Acoustic transducer with a backing containing unidirectional fibers and methods of making and using same

Abstract

The disclosure, in one aspect, provides an apparatus that includes an acoustic transducer and a backing in contact with a side of the transducer. The backing includes substantially unidirectional fibers in a matrix of a material that has high shear wave acoustic attenuation. The fibers contact the side of the transducer at an angle configured to convert substantially all of the compressional waves that enter the backing for the transducer into shear waves, which shear waves are then attenuated by the matrix of the backing material.

The invention claimed is: 
     
       1. An apparatus comprising:
 an acoustic transducer; and 
 a backing coupled to a side of the transducer at an interface, the backing including unidirectional fibers aligned along a common longitudinal axis, the unidirectional fibers contacting the interface so that the longitudinal axis is at a selected angle with respect to a plane perpendicular to the interface, wherein the selected angle is selected to be at or above a critical angle of incidence at which compressional waves arriving at the interface are converted to shear waves for absorption by the backing. 
 
     
     
       2. The apparatus of  claim 1 , wherein the critical angle of incidence is about 15 degrees. 
     
     
       3. The apparatus of  claim 1  further comprising a processor configured to process signals provided by the transducer to determine a property of interest of an earth formation. 
     
     
       4. The apparatus of  claim 1 , wherein the unidirectional fibers are tungsten wires embedded in a matrix material of the backing. 
     
     
       5. The apparatus of  claim 1 , wherein an acoustic impedance of the acoustic transducer substantially matches a perpendicular acoustic impedance of the backing at the interface between the backing and the transducer. 
     
     
       6. The apparatus of  claim 1 , wherein the backing includes packed fibers coated with a matrix material. 
     
     
       7. The apparatus of  claim 1 , wherein the acoustic transducer is pressure compensated for use in a wellbore. 
     
     
       8. An apparatus comprising:
 a tool configured for logging a wellbore, the tool comprising:
 an acoustic transducer configured to receive acoustic waves from an earth formation and provide signals responsive to the received acoustic waves, and 
 a backing coupled to a side of the acoustic transducer at an interface, the backing including unidirectional fibers in a matrix, wherein the unidirectional fibers are aligned along a common longitudinal axis and contact the interface so that the longitudinal axis is at a selected angle with respect to a plane perpendicular to the interface, wherein the selected angle is selected to be at or above a critical angle of incidence at which compressional waves received at the interface are converted to shear waves for absorption by the matrix; and 
 
 a processor configured to process the signals provided by the acoustic transducer to determine a property of interest. 
 
     
     
       9. The apparatus of  claim 8 , wherein an acoustic impedance of the acoustic transducer substantially matches a perpendicular acoustic impedance of the backing at the interface between the backing and the acoustic transducer. 
     
     
       10. A method of determining a parameter of interest, comprising:
 transmitting acoustic waves into a formation from a tool deployed in a wellbore; 
 receiving acoustic waves from the formation responsive to the transmitted acoustic waves at a transducer in the tool; 
 providing signals responsive to the received acoustic waves by the transducer; 
 converting compressional waves at the transducer into shear waves by a backing coupled to a side of the transducer at an interface, the backing including unidirectional fibers aligned along a common longitudinal axis, the unidirectional fibers contacting the interface so that the longitudinal axis is at a selected angle with respect to a plane perpendicular to the interface, wherein the selected angle is at or above a critical angle of incidence at which compressional waves arriving at the interface are converted to shear waves for absorption in a matrix of the backing; and 
 processing the signals provided by the transducer using a processor to determine the property of interest. 
 
     
     
       11. The method of  claim 10 , wherein the critical angle of incidence is about 15 degrees. 
     
     
       12. The method of  claim 10 , wherein the backing is coupled to the transducer by one of: a mechanical device, an epoxy, and a resin. 
     
     
       13. The method of  claim 10  further comprising pressure compensating the transducer. 
     
     
       14. The method of  claim 10  further comprising substantially matching an acoustic impedance of the transducer to a perpendicular acoustic impedance of the backing at an interface between the backing and the transducer. 
     
     
       15. The method of  claim 10  further comprising providing a protective window for the transducer and placing a material having a selected acoustic impedance between the window and the transducer. 
     
     
       16. The method of  claim 10 , wherein the backing includes packed fibers coated with a matrix material.